Floating applications in seamless mode is supported by the Microsoft's implementation of RDP only starting from version 5.1, but only a Windows Terminal Server can distribute applications.
As stated in the Microsoft technical documentation, when connecting to a Remote Desktop service (e.g. Windows XP Professional), every request for a single application is ignored:
A Specified Program Will Not Open If you are having problems opening a specified program on the host computer (on the Programs tab of the Remote Desktop Connection), you might be connecting to a computer running Windows XP Professional. Specified programs will open only when connecting to a terminal server, not when starting a Remote Desktop session. Remote Desktop provides access to the actual console session of the remote computer. You cannot specify programs that open in a Remote Desktop session. http://technet.microsoft.com/en-us/library/bb457106.aspx
Furthermore, since NoMachine 4 (and NX 3.5.0) relies on the rdesktop project to manage RDP sessions, support for RDP sessions in seamless mode is strictly related to rdesktop capabilities of supporting RDP 5.
If the rdesktop version you are using supports RDP 5.x, NoMachine can run the Windows application in seamless mode.